Had this problem once in a dragster tail shaft was bent.

I had this exact problem this year when I replaced trans and driveshaft after I blew my trans. and tore driveshaft in half. Had driveshaft checked for balance, etc., checked perfect. Asking around I found two local racers who had same problem and said U-joint bolts were too tight in their case. Checked with local driveline experts (and they ARE experts!) and they verified this was frequently the problem. Checking my rear U-Joint bolts I found my trans guys had installed one pair so tight I needed a breaker bar to loosen them and other pair was just over finger tight. I got new bolts and torqued them all to 35 ft/lbs and problem solved- no more vibration.

This^^^^

quote:

Originally posted by SS427IMP:
I had this exact problem this year when I replaced trans and driveshaft after I blew my trans. and tore driveshaft in half. Had driveshaft checked for balance, etc., checked perfect. Asking around I found two local racers who had same problem and said U-joint bolts were too tight in their case. Checked with local driveline experts (and they ARE experts!) and they verified this was frequently the problem. Checking my rear U-Joint bolts I found my trans guys had installed one pair so tight I needed a breaker bar to loosen them and other pair was just over finger tight. I got new bolts and torqued them all to 35 ft/lbs and problem solved- no more vibration.

I believe the torque value will depend on the size of the u-joint and the attachment device.

I have 1350 with a U-bolt and the included instructions said 15 ft lbs on the nuts.

If you look at Strange Engineering Heavy Duty U-Joint Cap Kit it lists the torque value as 35 ft lbs.

https://www.summitracing.com/p.../mor-85830/overview/

quote:

Place 3/8 Bolts through holes in Girdles and secure over u-joint bearing caps
with the Flange Nuts.
Tighten bolts evenly making sure that the Girdles come down straight and not
****ed to one side. Torque fasteners to 45 ft/lbs.

This is what I have on my car.

Update for 2019

Getting a very late start and Sunday was my first time out this year. It was hot and humid with a high chance of rain. I made 2 runs before the rains came and I am happy to report, I did not feel the vibration anymore.

Not sure what fixed it but glad it's gone.

-replaced all the bearing in the rear and stepped up to a 4.88 gear this time
-re-aligned the front end (4* caster, 0 camber, 1/16-1/8 toe in) it was out quite a bit from where it is now
-re-scaled the car (new exhaust and mufflers added 93 lbs)

Great news!

UPDATE:

Vibration came back the following race. Same spot @6400 in high gear but with the new lower gears, it was earlier in the run.

I was talked into changing the Fluidamper since it was on both engines. When I went to pull it off, it fell off. Was snugging up the puller and it never got tight. Turned it by hand and just grabbed the balancer and pulled it off the crank.

Went to install the new Superdamper and the installation tool broke off in the crank. (Just my luck right)

Pulled the engine out to remove the tool and go thru it. Sent the crank out to get it magged and cleaned up since you could see where the old damper was chattering on the snout.

Should be back together in a couple weeks I hope.

I’ve been told that just because the crank is magnafluxed doesn’t make it 100% good. I found this out the hard way when a crank (Eagle 1500 hp rated) I had with 500 runs was magged and installed in a new SHP sbc engine with 650 hp and broke at the 7/8 cheek and pushed a hole the size of my fist out the rear of this engine at 167 runs.

Not saying this could happen to you, just saying what I’ve experienced.

Makes sense. Magnaflux for surface indications (cracks) guess you would need Ultrasonic testing as well to have 100% but that gets pricey. Mag will find high percentage.